1. Field of the Invention
The present invention relates to a wire electric discharge machine configured to machine a workpiece with a voltage applied between the workpiece and a wire electrode, and more particularly, to a wire electric discharge machine and a wire electric discharge machining method, configured to machine a graphite or carbon-composite workpiece.
2. Description of the Related Art
Since graphite and carbon-composite materials are light in weight and excellent in electrical conductivity, heat-resisting property, thermal conductivity, and lubricity, they are frequently used for electrodes for electric discharge machining, slide bearings, etc. In cutting work for cutting a desired shape, a tool wears quickly and chips cannot be easily removed.
In the cutting work, as shown in FIGS. 1A and 1B and FIGS. 2A and 2B, a number of necessary shapes are very difficult to machine. Recently, wire electric discharge machining has been becoming noticeable as a method to machine those awkward shapes for cutting. FIGS. 1A and 1B are plan and perspective views, respectively, showing a machining shape with a rib width of 0.5 mm and plate thickness of 200 mm. On the other hand, FIGS. 2A and 2B are plan and perspective views, respectively, showing a machining shape with a rib width of 0.5 mm and plate thickness of 200 mm.
Generally, in wire electric discharge machining, a sharp current pulse with a high peak current value and short pulse width is used in rough machining of a workpiece by a wire electric discharge machine, as disclosed in Japanese Patent Application Laid-Open No. 11-48039. FIG. 3 shows a current pulse with a sharp leading edge. In this case, the machining supply voltage of the machine is so high that the normal peak current value used for machining ranges from about 300 to 1,000 amperes and the pulse width is about 0.5 microsecond.
Conventionally, however, it is difficult to efficiently quickly machine a brittle graphite or carbon-composite material into the thin rib-like shapes shown in FIGS. 1A to 2B with good surface quality by using a current pulse with a peak value of 300 to 1,000 amperes and a width of 0.5 microsecond.
If the current pulse for rough machining used in Japanese Patent Application Laid-Open No. 11-48039 is applied to machining of a porous, brittle graphite material, large fragments of about 0.2 mm are inevitably produced, as shown in FIG. 4, so that products with good surface quality cannot be obtained. FIG. 4 shows a machined surface photo 100 of the graphite material under conventional conditions. FIG. 5 is a graph showing machined surface roughness 104 on line 102-102 of FIG. 4.
In the case where the thin rib-like shapes are machined by the conventional wire electric discharge machine, as shown in FIGS. 1A to 2B, rib portions are inevitably damaged, as in machining examples shown in FIGS. 6A and 6B, so that a desired machining shape cannot be obtained. FIGS. 6A and 6B show a crack 107 of a thin rib 106 and a fragment 111 of a thin rib 110, respectively.
FIG. 6A shows how a wire electrode 1 is moved relative to the thin rib 106 in the direction of an arrow 108, thereby machining the reverse surface of the rib 106, and is then moved relative to the rib 106 in the direction of an arrow 109, thereby machining the obverse surface of the rib 106. Likewise, FIG. 6B shows how the wire electrode 1 is moved relative to the thin rib 110 in the direction of an arrow 112, thereby machining the reverse surface of the rib 110, and is then moved relative to the rib 110 in the direction of an arrow 113, thereby machining the obverse surface of the rib 110.
As disclosed in Japanese Patent Application Laid-Open No. 2008-260070, the defective part (the crack 107 or fragment 111 of the thin rib shown in FIG. 6A or 6B) may be repaired by a second-cut method. If the thin rib is damaged, however, it cannot be repaired by this second-cut method. Further, many of workpieces with thin rib-like shapes may be warped during rough machining so that rough-machined shapes are distorted. It is often difficult to correct the distortion in the machining by the second-cut method. Therefore, a method is used in which machining work is finished when rough machining is completed.
In machining the graphite material, moreover, graphite is sublimated by the heat of electric discharge to produce a lot of gas during the machining. Thus, in machining a workpiece formed of a thick graphite plate, in particular, the gas produced in the area of electric discharge reduces the effect of cooling of machining parts, especially the wire electrode, by a working fluid. In addition, there is a problem that the wire electrode is thermally fused to cause a wire breakage, which prevents continuation of the machining.